1. Field of the Invention
The present invention relates generally to power transmission in tracked vehicles such as tanks and more specifically to a drive system wherein a hydrostatic steering drive is employable to provide vehicle propulsion and steering control at low speeds.
2. Background Art
Drive assemblies for tracked vehicles having a mechanical speed change gear box employed in a propulsion drive with an infinitely variable superimposed steering drive have been disclosed in U.S. Pat. No. 4,428,445, U.S. Pat. No. 4,327,603 and U.S. Pat. No. 4,280,579. In addition, tracked vehicle drive systems including steering drives having hydrostatic or hydrodynamic units for driving a neutral shaft were disclosed in U.S. Pat. No. 3,371,734 and the following patent documents of the Federal Republic of Germany; DEP 14 80 725, DEP 860,601; DAS 19 29 380; DOS 23 22 457 and DAS 11 74 182.
The vehicle drive systems of the past were incapable of providing both high speed vehicle travel, in the order of, for example, 60 km/hr with precise steering control at such high speeds while maintaining the ability to accurately drive the vehicle at relatively slow speeds, for example, in the order of 0.2 to 5.0 km/hr as may be necessitated by special circumstances. In addition, prior drive systems lacked the ability to provide controlled driving in a straight direction without manual correction for ground conditions, for example, inclination or slope of the travel path or different driving resistances due to ground terrain and/or drive system component variations.
A further deficiency encoutered with prior drive systems related to control apparatus for steering and/or propulsion drives. In full track vehicles such as tanks, high demands were made of the power transmission systems including propulsion and steering drives as well as control apparatus for such drives. Such demands dealt primarily with efficiency and operation safety for both military reasons and accident avoidance. For safety purposes, it was desired to operate control elements through mechanical linkages yet control systems heretofore lacked the ability to provide mechanical steering linkage controls which could be automatically varied in position as a function of predetermined additional parameters.